Ideal filtration membranes possess adequate strength for the intended applications and combine high air or liquid permeability properties with high filtration efficiencies. High permeability or high flow through the membrane for a given pressure drop across the membrane affords lower energy costs due to lower energy loss and more rapid filtration times. Additionally, these features make it possible to install smaller, more cost effective systems. Improved filtration efficiency affords improved capture of contaminants. Typically, optimizing either filtration efficiency or permeability comes at the expense of compromising the other. For example, in order to capture smaller particles, the membrane must possess small pores, but the smaller pore size typically increases flow resistance and therefore decreases the liquid or gas permeability through the membrane. Similarly, increasing flow through the membrane is readily achieved by increasing the pore size of the filter media, but in doing so the membrane captures fewer particles and is less efficient in the capture of smaller particles.
Improvement in the development of filtration media has focused on finding ideal combinations of high permeability, small pore size, and high strength. Expanded PTFE (ePTFE) membranes have enjoyed great success in the fields of liquid and gas filtration. Expanded PTFE membranes are typically hydrophobic unless treated or modified or otherwise altered. Such membranes not only possess high chemical inertness and thermal stability at extremes of temperature, but they also possess high strength.
Methods for processing expanded PTFE and articles made therefrom are taught in U.S. Pat. No. 5,476,589 to Bacino. Bacino teaches very thin articles consisting essentially of microfibrils. This patent enables the processing of high strength PTFE articles possessing small pore sizes and high air permeability not previously obtainable. The articles are made by transversely stretching PTFE, then expanding it in the longitudinal and transverse directions. These materials, albeit having improved filtration performance compared to predecessor membranes, have limitations in their ability to satisfy increasingly demanding commercial needs.
It is well established in the art that increasing the ratio of expansion of PTFE typically increases the pore size of resulting porous expanded articles. The larger pore size results in lower flow resistance through the membranes, but, as noted above, at the expense of filtration efficiencies, especially for smaller particles. Further stretching tends to decrease membrane thickness which can result in a reduction in flow resistance.
Thus, while numerous efforts have been made to improve the filtration characteristics of PTFE membranes, a clear need still exists for thin, strong filtration membranes that provide both small pore size and low flow resistance.